You need the transceiver interface and interface cable part number 02118. The connection to the MB1 is via any of the methods described in the User Manual Addendum, depending on what, if any, other CAT enabled device you have in addition to the SDA.
A summary of those options:
1 Connection to the com port on the MB1 with CAT directed to it via either VSPE, SDC PA Tab or direct from the ESDR2 CAT Tab if you do not need to split com data (if not using a log book program for example)
2 A USB com port adapter from the MB1 with CAT data to it via one of the above methods
3 A MicroHam unit such as the MKII which has a dedicated CAT output for SteppIR